1. Field of the Invention
The invention relates to a method for generating a representation of the supply voltage in connection with service devices that are capable of feedback to a supply line, in particular for frequency converters of electric motors. The invention further relates to a circuit arrangement for executing the method.
2. Discussion of Related Art
Frequency converters for electric motors of machine tools are known from the prior art, which feed the energy, which is being generated in the course of braking the electric motors, back into the supply system. To feed the energy back into the power supply, the electric motors are operated in a generator mode. The voltage output by the electric motors is initially rectified and thereafter converted to a voltage that has the same frequency and phase position as the supply voltage. In a three-phase rotary current system, at least two such voltages must be selected for feedback, the third voltage can then be calculated from the two known ones. The voltages determined in this manner are called representations of the supply voltages.
In order to keep the outlay for wires as low as possible, for determining the representation of the supply voltage it is advantageous to pick up the required control signals from the supply system downstream of the commutating chokes, to which a signal, which is proportional to the supply voltage and is pulse-width-modulated, has been applied. Since the supply voltage has been modulated by the pulse width modulation at this point, a low pass filter is required for demodulating the supply voltage. This filter should rotate the phase as little as possible, but should demodulate the pulse width modulation as well as possible. A low pass filter of a critical frequency of approximately 2 kHz is used for this.
A disadvantage of such a filter is that harmonics of the supply frequency are not suppressed. The result of this is that in case of a connection to a supply system with a relatively high internal resistance (for example because of the use of a power supply transformer for matching the supply voltage), an oscillation tendency can easily occur. This applies in particular in feedback, since a positive feedback relative to the harmonics can occur.
These disadvantages are avoided when using so-called -90 degree filters. Two of three supply voltages of a rotary current supply system are fed to such a filter, the difference between the two is formed in an upstream-connected subtracter, and this difference is rotated by -90 degrees of angle by the filter. By means of this a representation of the third phase is generated. A -90 degree filter is a low pass filter of a very low critical frequency (for example 2 Hz), which generates a phase rotation of almost -90 degrees at 50 Hz. The exact rotation of over -90 degrees is achieved by a second filter of a critical frequency of approximately 1.25 kHz and connected in series. If no rotation of exactly -90 degrees is required, the second filter can also be omitted. This circuit has the advantage that all harmonics are satisfactorily damped out of the supply frequency because of the low critical frequency, so that therefore no sinusoidal voltage with a very low distortion degree is available.
However, the disadvantage of this circuit arrangement, also known from the prior art, lies in that the determination of the representation of the supply voltage is erroneous if a supply voltage fails, since the other two supply voltages are determined from the failed supply voltage. The result of this is that, if one of the three supply voltages fails, the phase current and the intermediate circuit voltage very quickly assume extreme values because of the erroneous triggering.